Wietse
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Liquid in the gauge (glycerine) is there to lubricate the mechanism and dampen gauge movement. Temperature does not effect accuracy on a gauge.
Fuel starvation issue?
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Liquid in the gauge (glycerine) is there to lubricate the mechanism and dampen gauge movement. Temperature does not effect accuracy on a gauge.
Korican
Member
This is from Summit Racing...
Notes:When reading fuel pressure, keep in mind that a fluid filled pressure gauge will change it’s reading as the temperature of the gauge case changes (Under hood temperature). As the liquid in the case heats or cools, it expands and contracts, causing case pressure to change as much as 1 psi for every 30 degrees temperature change of the gauge. Features 3/8 in. tube ends on the fuel block.
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Whoa...Really? I have one of those gauges. I got it because the old gauge shook so much, it was hard to read. I also have had a variance in pressure between a cold engine and a warm one. I'm talking a 75 degree engine as being cold and a 200 degree coolant reading as being up to temperature. That is 125 degrees difference. This could account for over 4 psi of difference according to Summit ?
Wietse
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Defines the quality of the gauge!
Korican
Member
Possibly and I am no expert with just going off things I have came across online with now what I have experienced on my own set up. Also do you have a fuel pressure regulator with that gauge that bounced. I ask because from my research that would typically happen because of not having a FPR to regulate the flow consisistenly which would allow the needle to stay stable.
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Korican
Member
Could be but also seems to be a general statement made which would leave me to believe it is more common to encounter than not. Seems regardless of "quality"...
Indeed, an "oil filled" gauge is tougher and more durable than a "dry" gauge, with the oil protecting the precision internals from shock and vibration, which explains why fewer and fewer dry gauges are offered for performance engines today. And there's no arguing that a liquid filled gauge looks like a higher quality instrument. Unfortunately, there's a hidden "gotcha" that users of this type of gauge experience, a reported lack of accuracy, or consistency, is common. In fact "oil filled" fuel pressure gauges do exhibit seemingly random pressure swings from 1-9 PSI. This is a fuel pressure "yellow-flag" for EFI engines running 40-60 PSI of fuel pressure, and a "black-flag" for carbureted engines running as little as 2-10 PSI.
The problem is one of physics, where the mechanism that moves the needle, called the Bourdon Tube, is submersed in a liquid (most commonly glycerin) that is used to fill the gauge case that houses it, and then sealed. The finished product is a beautiful, durable fuel pressure gauge that has a maddeningly mysterious tendency to "read all over the place". The most disconcerting aspect of this is nobody really sees the needle actually move, it just never quite seems to read the same pressure from one look to the next. Obviously this leads the owner or tuner to believe there must be something seriously wrong with their fuel system. Logically, most figure the regulator is the culprit and from there things can go downhill fast!
The anecdotal stories of bad experiences with oil filled gauges would literally fill a book. In fact there's rarely anything wrong with the regulator or anything else, it's just the gauge. You're about to see why, and more importantly, learn there is an oil filled gauge that resolves this problem, once and for all:
Bottom line, a common liquid filled pressure gauge is very susceptible to heat, changing its reading with the temperature of the gauge. Imagine that; engine bay temperatures can rise as much as 200-deg. Fahrenheit from cold to hot and we're using a fuel pressure gauge that changes it's reading with temperature... how does this happen? The simple explanation is when the glycerin used to fill the gauge heats up it expands, and being inside a sealed container (the gauge case), the internal case pressure rises. Internal gauge case pressure can increase as much as 1-9 PSI above atmospheric pressure when hot.
What difference does it make? Well, for every 1-PSI of pressure rise inside the case, the needle will fall 1-PSI. Of course, the higher the oil temperature, the higher the internal pressure, and the deeper the reading on the gauge falls. In extreme examples the result is like entering the "Twilight Zone"; take a carbureted engine with 8-PSI fuel pressure, and a hot gauge with 8 PSI of internal pressure, the pressure gauge went from reading 8 PSI to 0 (zero) PSI... is it any wonder racers get a little tense now and then?
Not every liquid filled gauge will get that hot and drop that far, but experience shows it's common for enough temperature rise to create the illusion of a 3-5 PSI drop. Few can ignore this, and so begin the dance of adjusting the regulator so the gauge reads right. Now, once we start chasing the gauge, well then pressure is all over the map, the engine is getting as angry as we are and we've gone from the frying pan into the proverbial fire. This issue with oil filled gauges is responsible for more frustration and confusion about fuel system performance than any other single thing experienced at Aeromotive!
https://www.verociousmotorsports.com/Articles/VM-Articles-Fuel-Pressure-Gauges-Liquid-Filled-vs-Dry
Brewzer67
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I've had my autometer gauges for 30+ years and remember when I bought them that there were instructions to leave the screw loosened on the top of the gauge to avoid the incorrect readings. They only recommended leaving the screw tightened if they were going in an off road vehicle.
Wietse
Well-Known Member
As above!
The gauges randomly sold in automotive shops are far far from good quality.
At work i use a lot of gauges (liquid filled) from the brand WIKA and are used on engine cooling systems in an engine room with a high ambient temperature and have never seen them fail due to temperature.
These gauges also come with instructions to vent the gauge case after installation and once in use.
This is just a rubber plug that will pop out very fast once pressure would build up inside.
Having pressure inside a gauge case is dangerous as many of them are equipped with glass that could blow up in your face.
A good accurate gauge will cost a decent amount, don't rely on any cheap gauges.
I had similar issues with fuel delivery issues on my 66 Charger 383 when I bought it. Same symptoms as others here have reported. Turned out to be the push rod. Appeared to be the original, car had 87.5 K miles when problem started. When measured against a new one, it was short by nearly 3/8", Yikes.. Only reason I knew what to look for were answered by searching many threads here and other Mopar forums.
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